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Math Help - compound interest problem(#of yrs): through log property.

  1. #1
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    Question compound interest problem(#of yrs): through log property.

    ok guys this is really bugging me. any help would be greatly appreciated.
    p-3500
    a-4000
    r-.0525
    quarterly rate.so n=4
    so the question is asking for the number of years. so t is my unknown variable.

    i set it up like this.

    4000= 3500 (1+.O525/4)^4t

    so far so good ryt?
    4000=3545.9375 ^4t

    using the property of log that can be written as

    log 4000= 4t log 3545.9375

    so ill get
    log 4000/log 3545.9375=4t

    1.01473=4t
    .2536=t

    my question is how come im not getting 2.5 something years. i need to multiply it by ten to get the real answer?
    is it because the base of log is 10 or am i doing something wrong.
    thanks

    im working on a 2nd problem
    this one im totally off
    a=10,000 p=5000 r=.09 n=2

    10000=5000(1+.09/2)^2t
    10000=5225^2t
    log10000=2t log 5225
    log10000/log5225=2t
    .5379=t

    the answer should be
    7.9
    thanks in advance guys
    Last edited by lutwey; July 31st 2010 at 04:41 PM.
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  2. #2
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    Quote Originally Posted by lutwey View Post
    ok guys this is really bugging me. any help would be greatly appreciated.
    p-3500
    a-4000
    r-.0525
    quarterly rate.so n=4
    so the question is asking for the number of years. so t is my unknown variable.

    i set it up like this.

    4000= 3500 (1+.O525/4)^4t

    so far so good ryt? yes, to the above point

    4000=3545.9375 ^4t this step is incorrect ... divide by 3500 first
    ...
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  3. #3
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    thanks for the quick response.
    but wats wrong about it?
    you cal always multiply them ryt?
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  4. #4
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    Quote Originally Posted by lutwey View Post
    thanks for the quick response.
    but wats wrong about it?
    you cal always multiply them ryt?
    order of operations ...

    if you had 5 \cdot 3^3 , would that equal 15^3 ???
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  5. #5
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    Hi lutwey,
    second problem
    10000=5000(1+.09/2)^2t
    Simplify
    2=(1+.045)^2t
    log 2= 2t log 1.045
    2t=log2/log1.045 =15.75
    t=7.87

    bjh
    Last edited by bjhopper; July 31st 2010 at 05:34 PM. Reason: spelling
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  6. #6
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    aahhhhhh!!! hahahahaha

    thanks skeeter and bj hopper.
    now i got it, been away from math for a couple of yrs.


    thanks a lot.
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